Patent GB-729088-A (GB)
Title: A prestressed article and method of, and apparatus for making the same
IPC: B28B23/04
Filed: 1953-02-20
Abstract: 729,088. Reinforced concrete. CROM, J. M. Feb. 20, 1953 [Feb. 20, 1952; Feb. 5, 1953], No. 4816/53. Class 20 (4). A concrete structural member, such as a slab or beam, is prestressed by winding wire under tension around the member, the wire being subsequently covered by an applied coat of mortar or gunite. The wire may be wound in a groove in the member, or partly in a groove and partly around the sides of the member. In Figs. 2, 3, the railway sleeper 20 is grooved at 21 to receive several turns of stressed wire 22 one end of which is anchored by being passed through a hole 24, the other end being held by a conventional split taper plug assembly 28 set in the applied mortar 26, or the tension in the wire may be held by bonding with the mortar which may be applied as the winding of the wire proceeds. The winding is effected by securing the sleeper, by bolts in the holes 29, to a turntable, the wire being pulled from a supply spool through a die which imparts the tension to the wire. If the member is too long to be revolved, it is held stationary and the wire-feeding and tensioning means is moved around it. A slotted plate 50, Fig. 7, is slipped over the wire and a split tapered anchor 57 is applied thereto to hold the wire in tension until the mortar sets, when the plate 50 and anchor 57 are removed. The bond between the wire and the mortar may be assisted by a conventional split-tapered anchor 63. The member, e.g. a beam 30, may be mounted on a horizontal revolving table 11 and the wire laid in channels 45, 46 which are connected by relatively narrow grooves 57, 58, deflectors 65, 66, 110, 111 being provided to guide the wires into the grooves 57, 58 at the middle of the beam, and to arrange them fanwise at the ends. The deflectors 65, 66 comprise frameworks 67, 68, 69, 71 positioned one at each end of the grooves 57, 58, the members 68, 69 being clamped against the sides of beam by a screwed crank 95. The curved guide surfaces 81, 82 lead the wire to plates 87, 88 having noses which project into the grooves 57, 58 and deliver the wires to the spaces between the said plates and the bottoms of the grooves. The bottom 61b of the groove may be inclined as in Fig. 12. The deflectors 110, 111 for spreading the turns of wire at the ends of the beam are mounted at the ends of the table 11, and comprise curved rails 110a, 111a pivoted at one end at 112 to fixed brackets 110, the middles of the rails being supported by arms 118, 119 pivoted at 120, 121 and raised or lowered by screw jacks 124. The jacks are operated, during the rotation of the table 11, by rollers 129, 129a engaging a stationary track 130. The wires may be retained in position, during the removal of the deflectors, by a clamping plate 154 secured by a spike which is driven into the side of the beam. The ends of the tensioned wire are temporarily held by a plate 156 and conventional cone and wedges anchors 164, 165, the wires then being protected and bonded by metal spraying, or by application of mortar &c. When the bonding material has hardened sufficiently, the anchors 156, 164, 165 are removed and the protruding ends of the wire clipped off, or the ends of the wire may be secured by driving wire-clamping spikes into the beam. The beam 30 may be a one-piece member or it may consist of a number of blocks which are bound together by the wire. The blocks, except the end blocks, may be hollow, and are held together temporarily, during the winding, by a bolt which passes through all the blocks. In a modification a slab having longitudinal grooves which form longitudinal ribs, is wound with wire about one rib, or about a rib and a side of the slab.